1. Field of the Invention
The present invention relates to a superconducting magnet comprising a refrigerator, and more particularly to a structure of a superconducting magnet capable of enhancing a refrigerating and assembling properties and of being composed in small size.
2. Description of the Related Art
FIG. 18 is a cross-sectional view showing an example of a conventional superconducting magnet. In FIG. 18, the numeral 1 designates a superconducting coil which is impregnated in a liquid helium 3 as a cryogenic refrigerant being filled in a helium vessel 2 as a cryogenic refrigerant vessel. The numeral 4 designates a vacuum vessel provided to surround the helium vessel 2 and vacuums the space between the vacuum vessel 4 and the helium vessel 2 for heat insulation.
The numerals 5 and 6 designate a second and a first heat-shields respectively, which are mounted in coaxial cylindrical form so as to surround the helium vessel 2 between the helium vessel 2 and the vacuum vessel 4, thereby reducing the heat invasion into the helium vessel 2.
The numeral 7 designates a liquid nitrogen recipient provided in a part of the second heat-shield 6 for storing a liquid nitrogen 8. The numeral 9 designates e.g. a Gihord Macmahon-type two-stage refrigerator including a first heat stage 10 of an absolute temperature 80 K. and a second heat stage 11 of 20 K. This refrigerator 9 is mounted vertically with respect to the axial direction of the magnet from upper side, and the first and the second heat stages 10 and 11 refrigerate respectively the first and the second heat-shield 6 and 5.
The numeral 13 designates a boat portion for injecting the liquid helium 3 and inserting a current lead for energizing the superconducting coil 1. The numeral 14 designates a constant temperature bore.
The refrigerator 9 used in the above-mentioned conventional superconducting magnet will now be described.
The refrigerator 9 is composed such that: a first-stage displacer 16 and a second-stage displacer 17 are slidably mounted in a cylinder 15 made of honing pipe and formed in two-stage; a first-stage seal 18 and a second-stage seal 19 for preventing any leakage of the helium gas are provided between the cylinder 15 and the first-stage displacer 16 and between the cylinder 15 and the second-stage displacer 17 respectively; and the first heat stage 10 and the second heat stage 11 are provided at the respective stage outer peripheral surface.
The first-stage displacer is provided with a first-stage cool heat accumulator 20 made by use of copper wire netting as a cool heat member, and the second-stage displacer 17 is provided with a second-stage cool heat accumulator 21 made by use of lead ball.
Further, the refrigerator 9 includes a helium compressor 25 for compressing helium gas 24 and gas ducts including an induction valve 26 and an exhaust valve 27 for inducing and exhausting the helium gas 24, and further includes a drive motor 28 for reciprocating the first and the second-stage displacers 16 and 17 in the cylinder 15 and driving the induction valve 26 and the exhaust valve 27 in synchronicity with the reciprocation.
The refrigerator composed as mentioned above will be operated in the following manner.
Firstly, in such a state as the first and the second-stage displacers 16 and 17 being at the lowest end and the induction valve 26 being opened and the exhaust valve 27 being closed, and the high-pressure helium gas 24 compressed by the helium compressor 25 is introduced into first and second-stage expansion chamber 22 and 23 and set in high-pressure state.
Next, the first and the second-stage displacers 16 and 17 moves upwardly, and in accordance therewith the high-pressure helium gas 24 is introduced into the first and the second-stage expansion chamber 22 and 23 through the first and the second-stage cool heat accumulators 20 and 21. During this operation, the induction valve 26 and the exhaust valve 27 does not move. The high-pressure helium gas 24 is refrigerated to a predetermined temperature by the cold heat member when passing through the first and the second-stage cool heat accumulators 20 and 21.
Subsequently, by the downward movement of the first and second-stage displacers 16 and 17, the low temperature/low pressure helium gas 24 pass through the first and the second-stage cool heat accumulators 20 and 21 to be exhausted through the exhaust valve 27. Then the low temperature/low pressure helium gas 24 refrigerates the cool heat member of the first and second-stage cool heat accumulator 20 and 21, and thereafter returns to the helium compressor 25. The exhaust valve 27 closes while the induction valve 26 opens, and the high-pressure helium gas 24 having been compressed by the helium compressor 25 is introduced, and the pressures in the first and the second-stage expansion chambers 22 and 23 changes from low-pressure state to high-pressure state.
Thus, by repeating the aforementioned operations, the first and the second-stage heat stages 10 and 11 are refrigerated to temperatures of 80 K. and 20 K. respectively.
Subsequently, the operation of the aforementioned conventional superconducting magnet will now be described.
The first heat shield 6 is refrigerated to 80 K. by the first heat stage of the refrigerator 9 and the liquid nitrogen 8 contained in the liquid nitrogen recipient 7. The second heat-shield 5 is refrigerated to 20 K. by the second heat stage 11 of the refrigerator 9. Any invading heat from outside is vacuum-insulated by the vacuum vessel 4 and is further shut by the first and the second heat-shields 6 and 5 so as to reduce the heat invasion into the helium vessel 2.
The superconducting coil 1 is cryogenically refrigerated (e.g. 4.2 K.) by the liquid helium 3 in the helium vessel 2 to hold its superconducting state, and receives energizing current from an external superconducting magnet power source (not shown) through a current lead (not shown) to generate a required magnetic field.
However, the aforementioned conventional superconducting magnet is a lateral hollow magnet and the refrigerator 9 is mounted vertically with respect to the axial direction of the magnet. Therefore, it is necessary to ensure a sufficient length for the reciprocating movement of the piston called displacer for achieving the refrigerating property of the refrigerator 9 and to establish a large space between the first heat-shield 6 and the second heat-shield 5 and the space between the vacuum vessel 4 and the first heat-shield 6, thereby increasing the height and the total size of the arrangement.
To cope with such problems, the following superconducting magnet is proposed.
FIG. 19 is a cross-sectional view showing another example of the conventional superconducting magnet disclosed in, for example, Japanese Patent Laid-Open No. Sho 63-164205. In this conventional superconducting magnet shown in FIG. 19, the refrigerator 9 is provided in parallel to the axial direction of the magnet over the vacuum vessel 4, and the first heat stage 10 and the second heat stage 11 are coupled through copper plates etc. to the first heat-shield 6 and the second heat-shield 5 respectively.
The conventional superconducting magnet thus composed has a smaller radial dimension by arranging such that the reciprocating direction of the dimensionally longest first and second-stage displacers 16 and 17 of the refrigerator 9 is in parallel to the axial direction of the magnet.
According to the conventional superconducting magnet, as mentioned above, since the refrigerator 9 is mounted vertically with respect to the axial direction of the magnet, the height of the magnet device is increased and the entire system becomes in large size.
Further, in the conventional superconducting magnet coping with such problems, although the radial dimension of the magnet device is reduced by arranging the refrigerator 9 in parallel to the axial direction of the magnet over the vacuum vessel 4, the helium vessel 2 cannot be directly refrigerated and the helium gas being evaporated from the liquid helium 3 due to the fact that the refrigerator 9 includes the first heat stage 10 of 80 K. and the second heat stage 11 of 20 K.